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| United States Patent |
5,346,760
|
|
Lemoine
, et al.
|
September 13, 1994
|
Composite material based on rubbers of the silicone type and of the
ethylene-propylene copolymer or terpolymer type
Abstract
A composite material comprising at least two elastomer layers, the first
layer having a first composition comprising from 40 to 80 percent by
weight of silicone rubber, the second layer having a second composition
comprising from 50 to 100 percent by weight of rubber of the
ethylene-propylene copolymer and/or terpolymer type, wherein the first
composition further contains from 20 to 60 percent by weight of rubber of
the ethylene-propylene copolymer and/or terpolymer type, and each of the
two compositions contains more than 5 percent, and preferably from 5 to 60
percent, by weight of silica.
| Inventors:
|
Lemoine; Guy (Le Havre, FR);
Benard; Etienne (Le Havre, FR);
Exandier; Christian (Le Fontenay, FR)
|
| Assignee:
|
Total Ruffinage Distribution S.A. (Levallois Perret, FR)
|
| Appl. No.:
|
824326 |
| Filed:
|
January 23, 1992 |
| Current U.S. Class: |
428/331; 428/328; 428/329; 428/330; 428/332; 428/334; 428/447; 428/451; 428/516; 428/517; 428/519 |
| Intern'l Class: |
B32B 005/16; B32B 025/08; B32B 025/16; B32B 025/20 |
| Field of Search: |
428/328,329,331,332,447,451,330,334,517,521,516,519
|
References Cited
U.S. Patent Documents
| 3398043 | Aug., 1968 | Youngs | 428/447.
|
| 4714733 | Dec., 1987 | Itoh et al. | 524/493.
|
| 4808643 | Feb., 1989 | Lemoine et al. | 524/87.
|
| Foreign Patent Documents |
| 0286551 | Oct., 1988 | EP.
| |
| 314396 | Oct., 1988 | EP.
| |
| 0310724 | May., 1992 | EP.
| |
| 1489261 | Jul., 1967 | FR.
| |
| 2597028 | Oct., 1987 | FR.
| |
| 56-116739 | Sep., 1981 | JP.
| |
| WO86/02088 | Sep., 1985 | WO.
| |
Primary Examiner: Nakarani; D. S.
Attorney, Agent or Firm: Safford; A. Thomas S.
Claims
What is claimed is:
1. A composite material comprising at least two different elastomer layers,
the first layer having a first composition of from 40 to 80 percent by
weight of silicone rubber and from 20 to 60 percent by weight of
ethylene-propylene copolymer and/or terpolymer rubber, the second layer
having a second composition whose rubber content consists essentially of
from 50 to 100 percent by weight of ethylene-propylene copolymer and/or
terpolymer, and from 0 to 50 percent by weight of a rubber selected from
the group consisting of nitrile rubbers, styrene-butadiene rubbers,
natural rubbers, fluoroelastomers, chlorosulfonated or chlorinated
polyethylenes, acrylic rubbers, epichlorohydrin rubbers, thermoplastic
elastomers and nonvulcanizable thermoplastics, the first composition
contains more than 10 percent by weight of silica and the second
composition contains more than 10 percent by weight of silica in said
compositions, and the silica has a pore volume ranging from 150 to 250
m.sup.3 /gram and an average primary particle size of less than 12
nanometers.
2. The composite material as defined in claim 1, wherein each of the two
compositions contains from 10 to 60% by weight of silica.
3. The composite material as defined in claim 1, wherein the silica used in
the two elastomer compositions is pyrogenic or precipitated silica.
4. The composite material as defined in claim 1, wherein the first
silicone-based layer contains less than 10 percent by weight of the
rubber, of hydrogenated oil.
5. The composite material as defined in claim 3, wherein the first
silicone-based layer contains less than 10 percent by weight of the
rubber, of unhydrogenated oil.
6. The composite material as defined in claim 4, wherein the first
silicone-based layer contains less than 1 percent by weight of the rubber,
of hydrogenated oil.
7. The composite material as defined in claim 5, wherein the first
silicone-based layer contains less than 1 percent by weight of the rubber,
of unhydrogenated oil.
8. The composite material as defined in claim 1, wherein the silicone
rubber of the first layer is a diorganopolysiloxane.
9. The composite material as defined in claim 5, wherein the silicone
rubber of the first layer is a diorganopolysiloxane.
10. The composite material as defined in claim 1, wherein the
ethylene-propylene copolymer and/or terpolymer rubber contained in the
first and second layers is an ethylene-propylene copolymer comprising (1)
about 40 to 80 percent by weight of ethylene and 60 to 20 percent by
weight of propylene, (2) from 40 to 80 percent by weight of ethylene, from
60 to 20 percent by weight of propylene, and from 1 to 10 percent by
weight of a conjugated monomeric diene or (3) a mixture of the two
copolymers (1) and (2).
11. The composite material as defined in claim 9, wherein the
ethylene-propylene copolymer and/or terpolymer rubber contained in the
first and second layers is an ethylene-propylene copolymer comprising (1)
about 40 to 80 percent by weight of ethylene and 60 to 20 percent by
weight of propylene, (2) from 40 to 80 percent by weight of ethylene, from
60 to 20 percent by weight of propylene, and from 1 to 10 percent by
weight of a conjugated monomeric diene or(3) a mixture of the two
copolymers(1) and (2).
12. The composite material as defined in claim 1, wherein the elastomer
layer based on silicone contains peroxide as a vulcanizing agent.
13. The composite material as defined in claim 1, wherein the layer based
on silicone rubber contains from 5 to 80 parts by weight of colorants
selected from the group consisting zinc oxide, iron oxide, titanium oxide,
cobalt oxide, magnesium oxide, aluminum oxide, organic pigment and any
other inorganic pigment, per 100 parts by weight of said first
composition.
14. The composite material as defined in claim 11, wherein the layer based
on silicone rubber contains from 5 to 80 parts by weight of colorants
selected from the group consisting zinc oxide, iron oxide, titanium oxide,
cobalt oxide, magnesium oxide, aluminum oxide, organic pigment and any
other inorganic pigment, per 100 parts by weight of said first
composition.
15. The composite material as defined in claim 1, wherein the first layer
based on silicone is used as a face layer of the composite material in a
thickness of less than 1 mm.
16. The composite material as defined in claim 14, wherein the first layer
based on silicone is used as a face layer of the composite material in a
thickness of between 0.1 and 0.5 mm.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a composite material comprising at least
two layers of elastomers, one layer containing a silicone-type rubber, and
the other layer an ethylene-propylene copolymer and/or terpolymer rubber.
The invention further relates to manufacturing processes for this
composite.
FIELD OF THE INVENTION
Elastomers of the silicone type have found many uses over the last few
years because of certain of their physical properties, such as their good
heat resistance, their good surface appearance, their anti-adhesive
character and their stability in the presence of colorants.
However, silicone-type elastomers are still rather expensive in comparison
with elastomers such as the ethylene-propylene copolymer and/or
terpolymer, which, moreover, have mechanical properties and a resistance
to certain chemical agents such as to oils and motor fuels which are
superior to those of silicone rubbers.
A first technique has been proposed with a view to rendering these two
elastomer types compatible in a single rubber formulation. (In this
connection, see Japanese patent 56 116 739 and European patent 286,551.)
However, in order that the hybrid material so obtained have characteristics
similar to those of silicone-type elastomers, it is necessary to
incorporate in it a significant quantity (at least 40 percent by weight)
of silicone-type rubber and, further, to render the two elastomers
compatible, for example, by grafting, which adds substantially to the
manufacturing costs. Moreover, it has long been attempted to make
composite materials which combine in one and the same material distinct
layers having the qualities inherent in each of these elastomers. However,
up to now it has not been possible to vulcanize a layer of a silicone-type
elastomer conjointly with a layer of ethylene-propylene copolymeric and/or
terpolymeric rubber. In fact, as taught in French patent 2,597,028, not
only will diffusion phenomena lead to the deterioration of the mechanical
properties of each of the layers of the composite but these layers are
insufficiently bonded to each other so that the risks of delamination
limit the uses of this type of composite material to only a few
applications, such as the fabrication of pipes, tubes and coaxial sheaths.
To overcome these drawbacks, it has been proposed to intercalate between
the layers of the different elastomers, an adhesive mixture (see French
patent 1,489,261) or a coupling medium (see French patent 2,597,028, cited
earlier) capable of serving as a protective barrier between the two
elastomer layers. However, problems which arise due to the cost and the
mechanical strength of the coupling medium and of the composite, limit the
industrial development of such composite materials.
SUMMARY OF THE INVENTION
The present invention provides a novel composite material comprising at
least two elastomer layers, one containing primarily a silicone-type
rubber and the other containing a rubber of the ethylene-propylene
copolymer and/or terpolymer type, which composite does not require an
intermediate protective material and which layers are nevertheless
sufficiently bonded to each other to prevent any subsequent delamination.
Applicants have, in fact, discovered that this advantageous result can be
surprisingly obtained by incorporating in each of the two layers of the
composite a certain quantity of a silica which silica has an appropriate
specific surface and particle size.
It is known that in order to render elastomers suitable for a given use, it
is advisable to incorporate in them in the course of their fabrication a
certain number of vulcanizing agents, plasticizers, processing aids,
additives, and especially reinforcing fillers.
The most widely used reinforcing fillers are, in particular, the carbon
blacks in the case of ethylene-propylene copolymers and/or terpolymers,
and the silicas in the case of silicone-based rubbers.
In the course of working with the fabrication and vulcanization of
composite materials comprising at least two layers of different
elastomers, Applicants have found that, contrary to the teaching of the
prior art, the two rubber layers can easily be covulcanized in a single
operation while preserving the desirable physical properties of each layer
and imparting to them perfect adhesion properties, provided that each of
the two layers contains at least:
Ethylene-propylene copolymer and/or terpolymer rubber in a substantial
quantity, meaning that the elastomer layer of the silicone type contains
from 20 to 60 percent by weight of ethylene-propylene copolymer and/or
terpolymer rubber.
A silica, preferably of a given porosity, in a quantity exceeding 5
percent, and preferably ranging from 5 to 60 percent, by weight of the
rubber, in each of the two elastomer layers, meaning that the layer of
ethylene-propylene copolymer and/or terpolymer contains preferably from 10
to 15 percent by weight of silica.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention thus has as a preferred embodiment a composite
material comprising at least two elastomer layers, the first layer having
a first composition which composition comprises from 40 to 80 percent by
weight of silicone rubber while the second layer has a second composition
which composition comprises from 50 to 100 percent by weight of rubber of
the ethylene-propylene copolymer and/or terpolymer type. The composite
material is further characterized in that the first composition also
contains from 20 to 60 percent by weight of rubber of the
ethylene-propylene copolymer and/or terpolymer type, and each of the two
compositions contains more than 5 percent, and preferably from 5 to 60
percent, by weight of silica.
While the patentability of the instant invention is not bound to this
concept and while explanation may not be the only one which explains the
improvement of the adhesion between the individual elastomer layers, it is
believed that the silica particles close to the interface of the two
layers develop the cohesion between the elastomers of each of these layers
and, during vulcanization, fix the elastomers of the ethylene-propylene
copolymer and/or terpolymer type of each layer both mechanically and
chemically. It appears, in fact, that through the selection of a certain
size, the structure of the particles and the chemistry of the surfaces,
the silicas may provide for the creation of a network and contribute to
the formation of more or less strong linkages between polymers, which may
be reinforced by the presence of various carefully selected coupling
agents.
To this end, the silicas used in the two elastomer layers in accordance
with the present invention are preferably so-called pyrogenic or
precipitated silicas having a pore volume (macroporosity) of from 150 to
250 m.sup.3 /g, as determined by the CTAB (cetyltrimethylammonium bromide)
method.
Moreover, these silicas may advantageously have a specific surface of from
175 to 225 m.sup.2 /g, as measured by the BET method.
In addition to the significant improvement in the adhesive power of the two
elastomer layers relative to each other, a second advantage inherent in
the choice of the silica stems from its particle size. A silica of small
particle size allows for very good dispersion to be obtained at a low
concentration. Furthermore, the increase in the viscosity of the resins
due to the introduction of such silicas is largely offset by the
plasticizing and dispersing effect of the polysiloxane gum introduced into
the silicone-type layer. This makes it possible to significantly limit the
quantity of oil in the formulation, which greatly minimizes the
possibility of migration of one layer into the other before the
vulcanizing stage. The physical properties of the final composite, such as
its lightfastness, its surface appearance, and the ease with which it can
be applied as a very fine film, are significantly improved.
The quantity of oil present in the silicone-rubber layer can thus be
limited to less than 10 percent by weight, and the use of oil can
preferably even be dispensed with through proper choice of a silica
having, for example, an average primary-particle size of less than 12
nanometers (nm), and of a polysiloxane gum.
In accordance with the invention, the layer of silicone-type elastomer has,
for example, a composition as follows (expressed in parts by weight):
(A.sub.1) From 40 to 80 parts of a diorganopolysiloxane gum of the general
formula R.sub.3-a (R'O).sub.a SiO(R.sub.2 SiO).sub.n Si(OR').sub.a
R.sub.3-a, where the symbols R, alike or different, represent C.sub.1
-C.sub.8 hydrocarbon radicals, substituted or unsubstituted by halogen
atoms, or cyano radicals; the symbol R' represents a hydrogen atom or a
C.sub.1 -C.sub.4 alkyl radical; the symbol a represents 0 or 1; and the
symbol n represents a number having a sufficient value to obtain a
viscosity of at least 1 million mPa's at 25.degree. C., at least 50
percent by number of the radicals represented by R being methyl radicals,
and from 0.005 to 0.5 mole percent of the units entering into the
composition of the gum being selected from among those of the formulas
(CH.sub.2 .dbd.CH) (R)SiO and (CH.sub.2 .dbd.CH)R.sub.2-a (R'O).sub.a
Si.sub.0.5 ;
(B.sub.1) from 60 to 20 parts of an ethylene-propylene copolymer and/or
terpolymer gum comprising from 40 to 80 percent by weight of ethylene,
from 60 to 20 percent by weight of propylene, and from 1 to 10 percent by
weight of a conjugated monomeric diene, or a mixture of these two
polymers, and especially an elastomer such as EPDM (ethylene-propylene
diene monomer);
(C.sub.1) from 5 to 60 parts of a silica selected preferably from among the
pyrogenic or precipitated silicas;
(D.sub.1) from 5 to 80 parts of other, nonreinforcing fillers;
(E.sub.1) from 0 to 5 parts of compounds having antioxidant action;
(F.sub.1) less than 10 parts, and preferably less than 1 part, of an
organic or petroleum-base oil of a type known per se;
(G.sub.1) from 2 to 10 parts of a crosslinking agent of a type known per
se; and
(H.sub.1) from 0 to 25 parts of crosslinking coagents.
As component (A.sub.1), a single diorganopolysiloxane of the type defined
above or a mixture of such compounds may be used.
Component (B.sub.1) is well known in the art and is commercially available
in the unvulcanized state. The EPDM may be used alone or in a mixture.
Generally, component (B.sub.1) contains various known additives, such as
reinforcing fillers or extenders, plasticizers, pigments and flame
retardants. However, the quantities of component (B.sub.1) to which
reference is made in this specification apply only to the polymers and do
not include additives.
The silicas (C.sub.1) may be incorporated as is or after treatment with
organosilicic compounds customarily used for this purpose. These compounds
include the methylpolysiloxanes, such as hexamethyldisiloxane or
octamethylcyclotetrasiloxane, methylpolysilazanes such as
hexamethyldisilazane or hexamethylcyclotrisilazane, chlorosilanes such as
dimethyldichlorosilane, trimethylchlorosilane, methylvinyldichlorosilane
or dimethylvinylchlorosilane, alkoxysilanes such as
dimethyldimethoxysilane, dimethylvinylethoxysilane or
trimethylmethoxysilane. During this treatment, the silicas may undergo a
weight increase of up to about 18 to 20 percent of their starting weight.
These silicas preferably have a specific surface, a macroporosity and a
particle size as described earlier.
The fillers (D.sub.1) consist of mineral fillers whose average particle
diameter is greater than 100 nm. They include, in particular, ground
quartz, calcined clays, and calcium carbonate, colorants such as the zinc,
iron, titanium, cobalt, magnesium and aluminum oxides, or inorganic or
organic pigmentary colorants (such as phthalocyanine, for example), carbon
black, the magnesium and aluminum silicates, the aluminum, calcium and
barium sulfates, asbestos, glass and carbon fibers as well as synthetic
fibers such as aramid, polyester, polyamide and rayon.
As compounds (E.sub.1), phenol derivatives, in particular, may be used.
The petroleum-base oil (F.sub.1) will be an optionally hydrogenated
paraffinic or naphthenic oil, or also a polyacid ester or another acid or
anhydride derivative.
The crosslinking agent (G.sub.1) is, in particular, an organic peroxide
such as bis 2,4-dichlorobenzoyl peroxide, dicumyl peroxide,
2,5-dimethyl-2,5-bis (tributylperoxy) hexane, or bis (tert-butyl peroxy
isopropyl) benzene. However, crosslinking may also be brought about by
radiation or by grafting of silanes which crosslink in moisture.
The crosslinking coagents (H.sub.1) are polybutadienes or
polymethacrylates, for example, used alone, as a mixture, or modified.
The layer of the composite based on an ethylenepropylene copolymer or
terpolymer preferably has the following composition:
(A.sub.2) from 50 to 100 parts of a rubber gum;
(B.sub.2) from 0 to 50 parts of a rubber different from rubber (A.sub.2);
(C.sub.2) from 5 to 60 parts of a silica;
(D.sub.2) from 10 to 200 parts of another filler, whether reinforcing or
not;
(E.sub.2) from 0 to 5 parts of at least one antioxidant compound;
(F.sub.2) from 0 to 150 parts of a petroleum-based or organic oil;
(G.sub.2) from 1 to 10 parts of a crosslinking agent; and
(H.sub.2) from 0 to 10 parts of products adapted to facilitate, in a known
manner, the processing of the composite; and
(I.sub.2) from 0 to 25 parts of crosslinking coagents.
The rubber gum (A.sub.2) is preferably of the same type as the gum chosen
as ingredient (B.sub.1) of the silicone-based composition described
earlier. This gum is advantageously a rubber of the EPDM type.
The rubber (B.sub.2) may be selected from among the vulcanizable
elastomers, such as the nitrile rubbers, styrenebutadiene rubbers or
natural rubber, the fluoroelastomers, the chlorosulfonated or chlorinated
polyethylenes, the acrylic rubbers, the epichlorohydrin rubbers and,
generally, all vulcanizable elastomers having properties which complement
those of the rubber (A.sub.2), as well as from among the thermoplastic
elastomers which do not require vulcanization and the nonvulcanizable
thermoplastics.
The silica (C.sub.2) is preferably selected from among the pyrogenic or
precipitated silicas of the same type as the silica chosen as ingredient
(C.sub.1) of the silicone-type elastomer layer and is preferably identical
with it.
The filler (D.sub.2) is preferably carbon black but may also be a filler
such as (D.sub.1) defined above.
The derivative (E.sub.2) is a high-molecular-weight phenol derivative, for
example.
The oil (F.sub.2) may be of a known type, for example, a paraffinic oil,
either hydrogenated or not.
Similarly, the crosslinking agent (G.sub.2) may be of a known type, for
example, a peroxide such as mentioned above for the agents (G.sub.1) of
the silicone elastomer layer, or a sulfur derivative selected for its
nonmigratory character.
Component (H.sub.2) is, for example, a polyethylene wax, a fatty acid
derivative or a glycerol customarily used in this type of composition.
The crosslinking agent (I.sub.2) may be a polybutadiene or a
polymethacrylate, alone, in a mixture, or modified.
The preparation of the compositions which make up each of the two elastomer
layers may be carried out in the usual manner by means of a Banbury mixer
or an in-line blender. Of course, care should be taken in their
preparation that the temperature during the mixing step is always below
the activation temperature of the vulcanizing agent, if such agents are
introduced during this step. The two compositions necessary for
fabrication of each of the two layers can then be covulcanized in contact
with each other through a heat treatment, preferably under pressure. The
vulcanization should be performed at a temperature of from 100.degree. to
200.degree. C. for a period of from three minutes to three hours at a
pressure ranging from 2 to 20 MPa. This may be done using salt baths,
fluidized beds, hot-air treatments or ultrahigh frequency systems in a
known manner.
The composite material in accordance with the invention may be prepared
conventionally by molding, compression, calendering or injection of the
two layers and their covulcanization in contact with each other.
This composite material may also be fabricated industrially in a
particularly advantageous manner by coextrusion. In fact, the choice of
the relative proportions of silicone, silica and EPDM described earlier
assures the compatibility and adhesion of the two layers without the use
of a third for compatibility or of another coupling system, so that the
composite material can be fabricated industrially directly by coextrusion
on a twin-screw extruder, followed by covulcanization.
The present invention is illustrated by the examples which follow. These
are selected and included for purposes of illustrating the invention in
order that others skilled in the art will more fully understand the
invention and the principles thereof and will thus be enabled to modify it
in a variety of forms, each as may be best suited the conditions of a
particular use.
EXAMPLE 1
A number of specimens of two-layer rubbers are produced under the following
conditions:
A sheet of silicone rubber is prepared from a test piece in a thin layer
about 1 mm thick, selected with a view to making an outer layer having
good gloss, slip, ozone resistance, lightfastness and color stability
properties. In fact, it is well known that unlike their homologs based on
ethylene-propylene copolymer and/or terpolymer rubbers, white silicone
rubbers do not yellow.
A thick sheet (about 1 cm thick) of ethylene-propylene copolymer rubber is
then prepared from a test piece based on EPDM, selected especially because
of its good mechanical properties and its moderate cost.
These two sheets are then covulcanized in a press in contact with each
other under the following conditions:
Duration: 4 to 5 minutes
Pressure: 50 to 100 bars
Temperature: 150.degree. to 180.degree. C. (depending on the nature of the
peroxide)
The various formulations of the compositions making up the two layers, as
well as the properties of the composite resulting from their
covulcanization, are given in Table I below in order to show the relative
amounts of the various components of each of the layers.
In this table, the silicone elastomer used is polysiloxane supplied by
Rhone-Poulenc as SIL 11, while the ethylene-propylene copolymer and/or
terpolymer elastomer is an EPDM furnished by Total Chimie under the
reference EP Total 035. Moreover, the silica used is a pyrogenic silica
with a macroporosity (CTAB) of 200 m.sup.2 /g, a specific surface of 200
m.sup.2 /g and a particle size of 12 nm. This silica is marketed by
Degussa as Aerosil 200.
The compositions listed in this table are given in parts by weight per 100
parties of gum (EPDM gum and/or silicone gum).
TABLE 1
__________________________________________________________________________
Composite E.sub.1
E.sub.2
E.sub.3
E.sub.4
E.sub.5
E.sub.6
E.sub.7
E.sub.8
E.sub.9
__________________________________________________________________________
Silicone rubber
Silicone gum
100 100 90 80 60 60 60 50 40
EPDM gum 0 0 10 20 40 40 40 50 60
Silica* 28 28 25 32 32 17 17 24 12
TiO.sub.2 10 10 10 10 10 10 10 10 10
ZnO 10 10 10 10 10 10 10 10 10
Paraffinic oil
0 0 0 0 0 0 15 0 0
Silane 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5
Peroxide 7 7 7 7 7 7 7 7 7
EPDM rubber
EPDM gum 100 100 100 100 100 100 100 100 100
Silica* 10 0 5 10 10 0 10 15 15
Carbon black
130 130 130 130 130 130 130 130 130
Paraffinic oil
80 80 80 80 80 80 80 80 80
ZnO 5 5 5 5 5 5 5 5 5
Peroxide 7 7 7 7 7 7 7 7 7
Properties of composite
Appearance ++ ++ ++ ++ ++ ++ ++ ++ +
Adhesion - - - + ++ - - ++ -
Peel strength
0 0 0.5 >2 >2.5
0 0.5 >3 1
(daN/cm)
__________________________________________________________________________
*Total quantity of silica in the elastomer layer.
It is apparent from this table that the outward appearance of the
silicone-based layer is not really satisfactory unless that layer contains
at least 40 percent by weight of silicone. Moreover, to obtain good
adhesion, with a peel strength of greater than 1.5, and preferably greater
than 2, two conditions must be satisfied. The EPDM-type rubber content
should exceed 20 percent, and the silica content of the EPDM-based layer
should exceed 10 percent, by weight of the rubber.
EXAMPLE 2
A number of specimens of two-layer rubber are prepared under the same
conditions of covulcanization in a press, with the two layers in contact
with each other, as in Example 1, from the same formulations as those of
specimen E.sub.8 given in Table 1 but by varying the grades of the silicas
used. The latter were supplied by Rhone-Poulenc (Zeosil) and Degussa
(Aerosil).
Table 2 which follows shows the development of the adhesion of the two
layers of the composite as a function of the grade of the silica.
TABLE 2
______________________________________
Composite E.sub.10 E.sub.11 E.sub.12
______________________________________
Silica Aerosil 200
Zeosil 35 Zeosil 175
Macroporosity (m.sup.2 /g)
200 90 185
Average particle
12 30 200*
size (nm)
Adhesion ++ - +
(daN/cm) >3 0.5 1.5
______________________________________
*Beads: 200 nm at the aggregate level.
It is apparent that while maintaining the macroporosity above about 150
m.sup.2 /g is a determinant factor for obtaining good adhesion, the
particle size also becomes important as that limit is approached (see
composite E.sub.12). Thus, with 200-nm silica beads, the adhesion is
barely satisfactory even though the macroporosity is 185 m.sup.2 /g.
EXAMPLE 3
This example is intended to illustrate the desirability of a composite
material in accordance with the invention, whose silicone-based rubber
layer is a very thin faced layer, which layer adheres to another layer of
EPDM-type rubber selected primarily on the basis of its intrinsic physical
properties and its cost.
The formulation of the two layers is the same as that of test piece E.sub.8
of Example 1. In this example, the composite material is fabricated by
coextrusion with a twin-screw extruder, and because of the excellent
serviceability of the silicone-based elastomer layer being treated (the
Mooney viscosity prior to coextrusion is about 40), it is indeed possible
to adhere a very thin white faced layer based on silicone rubber to a
black EPDM section.
The section E.sub.13 so prepared is exposed for about 1,000 hours to light
(xenon-arc-lamp Xenotest). In Table 3 which follows, the result of this
test is compared with that obtained with two control specimens subjected
to the same Xenotest. One of these, T.sub.1, is a specimen based on
silicone rubber of the same type as the face layer though not
covulcanizable since it does not incorporate EPDM. The other, T.sub.2, is
a specimen of white EPDM rubber.
TABLE 3
______________________________________
Composite E.sub.13 T.sub.1
T.sub.2
______________________________________
Lightfastness 8 8 2
after 1,000 hours
______________________________________
It is apparent from this table (tests E.sub.13 and T.sub.1) that the face
layer has the same insensitivity to the action of light as a pure silicone
rubber but in addition has the compatibility and adhesion properties
described above in Example 1. It is further observed that this thin faced
layer provides the inventive two-layer material with a superior esthetic
quality (see columns E.sub.13 and T.sub.2), since the white section based
on pure EPDM yellows considerably when subjected to the same light
treatment, which renders it unfit for certain industrial uses.
The instant application is based on French patent application No. 90.13531,
filed Oct. 31, 1990 and upon corresponding European patent application No.
91402926.9, filed Oct. 31, 1991, the disclosure of which Applicants hereby
incorporate by reference, although not claiming priority therefrom.
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